Such a mass-flow meter is known, for instance from U.S. Pat. No. 4,100,801. The principle on which the operation of this known mass-flow meter is based is as follows. The gas or liqid (the medium) of which the mass flow rate is to be measured, flows through a metal, heat conductive tube, of which the beginning and the end are kept at the same temperature. In the middle of the tube, inside or outside, a heating element is put, with which the medium and the tube are heated. On both sides of the heating element, temperature sensors are put, symmetrical in relation to this element.
When the medium in the tube does not flow, such a temperature profile occurs, when heated by the heating element, that in the middle of the heatiang element the temperature is at its maximum. Without heat loss to the environment, the tube temperature decrease is linear, as a function of the distance to the middle mentioned. Without heat loss, or with a heat loss to the environment which is symmetrical in relation to the middle mentioned, the temperature profile along the tube is symmetrical in relation to the middle mentioned.
The temperature sensors, placed symmetrically in relation to the heating element, in case of heating an immobile medium, show, in theory, a temperature difference equal to zero.
When the medium in the tube does flow, when heating in the way mentioned above takes place, a sensor upstream from the heating element will show a lower temperature than sensor which is placed symmetrically in relation to the first, downstream from the heating element. The difference in the temperature measured by both sensors is a measure for the speed of flow of the medium and therefore for the mass flow through the tube.
The mass-flow meter with temperature sensors, according to the principle mentioned above, shows a number of disadvantages.
Air currents on the outside of the tube can disturb the temperature profile along the tube wall, and thus can have a negative influence on the accuracy and reproduceability of a measurement. The phenomenon of air currents leading to a changed temperature profile is called external convection. External convection can be counteracted by isolating the tube thermally, for instance by wrapping it in a synthetic foam.
Another disadvantage is the internal convection, occuring in the form of gravity-induced convection flows in the medium itself, which can occur when the parts of the tube on both sides of the heating element cannot be oriented symmetrically in relation to the direction of gravity. Internal convection also provides cause for additional temperature gradients along the tube surface, and thus for extra inaccuracy in measuring the mass flow. This problem, which occurs especially when the mass-flow meter with temperature sensors is applied in spacecraft, can be counteracted by putting a loop course in the tube, because of which the medium is forced to turn around the flow direction at least once. A description of a mass-flow meter with loop-shaped tube is given in the mentioned U.S. Pat. No. 4,100,801.
Internal convection as well as external convection are manifest in the occurrence of so-called zero-offset, by which is meant the phenomenon that when the tube, with an immobile medium, is heated, in variance with that which one would expect from theory, temperature sensors applied symmetrically in relation to the heating element show a temperature difference not equal to zero. Internal and external convection are not the only causes for the occurrence of a zero-offset.
Other causes for the occurrence of a zero-offset are, among others, a difference in the temperature at the beginning and end of the tube, with an otherwise exact symmetry of the temperature sensors in relation to the heating element, a positioning of the temperature sensors not exactly symmetrical in relation to the middle of the tube, a positioning of the heating element not exactly in the middle of the tube and inhomogeneities in the tube wall.
In the known embodiments of the mass-flow meter with temperature sensors, the problem of occurring zero-offset is avoided by compensating for the zero-offset electronically. The disadvantage of this is the inherent need for cost-raising additional electronic equipment and/or components for reading the temperature sensors.